The invention concerns an optically nonlinear semiconductor material, a method for the production thereof, and its use.
Optically nonlinear materials are utilized in many applications today. In the optical information processing, they are, e.g., utilized for switching light by means of light. In the optical communication field, they can serve to clean signals from interfering noise, which is caused, for example, by amplified spontaneous emission (ASE). A further field of application is laser physics, where materials like this are utilized as saturable absorbers for passive mode locking in laser resonators for the purpose of generating ultrashort laser pulses (in the femto- or picosecond range). The passive mode locking, for example, can be achieved by the utilization of a mirror with saturable absorbers made of semiconductor materials (semiconductor saturable absorber mirror, SESAM) as resonator mirror (refer to. U. Keller et al., xe2x80x9cSemiconductor Saturable Absorber Mirrors (SESAM""s) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasersxe2x80x9d, IEEE Journal of Selected Topics in Quantum Electronics, volume 2, No. 3, September 1996). The reflectivity of a SESAM mirror of this type is higher at high light intensities because of absorption bleaching. A SESAM typically consists of a reflecting substrate, a saturable semiconductor absorber structure and, optionally, of an additional reflection or anti-reflection layer.
In applications such as the ones above (apart from others), the material characteristics
(a) response time,
(b) absorption modulation and
(c) non-saturable absorption losses
play an essential role and therefore can be designated as key parameters. The following demands are made of these key parameters of optically nonlinear materials:
(a) the response time should be adaptable to the corresponding application (for example, should lie in the pico- or femto-second range);
(b) the absorption modulation should be high;
(c) the non-saturable absorption losses should be low.
Up until now, no material is known, which simultaneously fulfills all these requirements in an ideal manner. In practice, the goal is to find a material, which fulfills the requirements as well as possible. Frequently a measure for the improvement of one key parameter leads to a deterioration of a different key parameter. If the occasion arises, therefore for a certain application a not completely satisfactory, but acceptable compromise between partially opposing material characteristics has to be reached.
By the term xe2x80x9cresponse timexe2x80x9d, in this document that time is understood, during which the initially rapid change of the optical material characteristics recovers mainly by charge carrier trapping (trapping). Apart from this, the optical material characteristics are also influenced by further, in most instances slower mechanisms.
Mentioned here as an example for a known optically nonlinear material shall be gallium arsenide (GaAs). The preferred method for the production of GaAs is the molecular beam epitaxy (MBE). Normally GaAs is grown at temperatures between 500 and 800xc2x0 C. This normal growth provides almost ideal stoichiometric crystals with high absorption modulations, low non-saturable absorption losses, but long response times (in the region of 100 ps).
In order to eliminate the disadvantage of long response times, the GaAs can also be grown at low temperatures of approx. 180 to 500xc2x0 C. With this low temperature process, non-stoichiometric crystals with a high crystal defect density are produced. The crystal defect density and therefore also the low temperature process can be determined or identified with the help of the near infrared absorption (NIRA) or the magnetic circular dichroism of absorption (MCDA) (refer to, e.g., X. Liu et al., xe2x80x9cMechanism responsible for the semi-insulating properties of low-temperature-grown GaAsxe2x80x9d, Appl. Phys. Lett. 65 (23), Dec. 5, 1994, page 3002 ff.). In actual fact, with the low temperature process one achieves adjustable, short response times (in the range of sub-pico-seconds up to several 10 ps); these advantages, however, have to be paid for by a low absorption modulation and high non-saturable absorption losses.
It is the objective of the invention to create an optically nonlinear semiconductor material, which simultaneously has influenceable response times, high absorption modulations and low non-saturable absorption losses. It is furthermore the objective of the invention to indicate a method for the production of such a material.
The objective is achieved by the material in accordance with the invention and by the method in accordance with the invention, as these are defined in the independent claims.
Surprisingly it was established, that the key parameters (a)-(c) mentioned above of an optically nonlinear semiconductor material grown at low temperatures, such as, e.g., GaAs, can be significantly improved, i.e., by up to one order of magnitude, by means of the following measures:
(i) the addition of foreign atoms (doping) and/or
(ii) additional thermal annealing.
Semiconductor materials, during the production of which at least one of these measures was undertaken, combine astonishingly favorable nonlinear optical material characteristics and in this way come close to an optimization of the key parameters in a manner not achieved up until now. They have in particular
(a) influenceable response times, as well as simultaneously
(b) high absorption modulations (comparable with those of normally grown semiconductor materials) and
(c) low non-saturable absorption losses (comparable with those of normally grown semiconductor materials).
For this reason, they are eminently suitable for nonlinear optical applications, in particular in the optical information processing field for the ultrafast switching of light by means of light, in the optical communication field for the cleaning optical signals from noise or in ultrashort pulse laser physics as saturable absorbers for lasers emitting ultrashort pulses. In the latter field of application, the semiconductor material in accordance with the invention is especially suitable for mirrors with at least one saturable absorber made of this semiconductor material.
Utilized as semiconductor material is in preference a III-V semiconductor, for example gallium arsenide (GaAs), indium-gallium arsenide (InGaAs), aluminium-gallium arsenide (AlGaAs) or indium-gallium arsenide-phosphide (InGaAsP). The semiconductor material is preferably produced by means of molecular beam epitaxy (MBE). Another possible production method is the gas phase deposition, in particular the metalorganic chemical vapor deposition (MOCVD).
In the case of a first method variant in accordance with the invention, a semiconductor material is produced at temperatures between 180 and 500xc2x0 C. and doped with foreign atoms. The foreign atoms in preference are at least one acceptor material, for example, beryllium (Be). The doping in preference takes place during the epitaxial growth of the semiconductor material in an ultra-high-vacuum chamber in the molecular beam. The foreign atom concentration is adjusted through the ratio of the molecular beam flow, for example, from Be to Ga and As. A doping of this kind can be identified subsequently, e.g., by means of the secondary ion mass spectroscopy (SIMS). Typical Be concentrations are between 1017 cmxe2x88x923 and 1020 cmxe2x88x923.
In the case of a second method variant in accordance with the invention, the semiconductor material is produced at temperatures between 180 and 500xc2x0 C. and subsequently thermally annealed. The annealing can take place during a minimum of 5 minutes at temperatures between 400 and 800xc2x0 C., or else also as rapid thermal annealing (RTA) during, for example. 10 sec at approx. 600 to 1000xc2x0 C. The thermal annealing in most instances leads to a certain precipitation of a semiconductor component; in the case of GaAs, for example, small As balls with diameters in the nanometer range are formed, typically between 2 and 10 nm, with a density of 1017 to 1018 cmxe2x88x923. (refer to, e.g., M. R. Melloch et al., xe2x80x9cFormation of arsenic precipitates in GaAs buffer layers grown by molecular beam epitaxy at low substrate temperatures, Appl. Phys. Lett. 57 (15), Oct. 8, 1990, page 1531 ff.); also a greater bandwidth of the density, for example from 1015 to 1019 cmxe2x88x923, is possible. If it is a III-V semiconductor material containing As, then the thermal annealing in preference takes place in an As atmosphere, in order to prevent a displacement of As out of the semiconductor or to at least reduce it.
The doping with foreign atoms and/or the thermal annealing leads to a significant improvement of the key parameters of the semiconductor material. These measures during the production in accordance with the invention have the effect, that the response times are significantly shorter than in the case of semiconductor materials, which have been produced at low temperatures without doping and without thermal annealing; nonetheless, the absorption modulation remains high and the non-saturable absorption losses low.
The favorable effects of the Be doping and/or of the thermal annealing can be explained by means of the following model (refer to the model for undoped GaAs grown at low temperatures in, e.g., U. Siegner et al., xe2x80x9cUltrafast high-intensity non-linear absorption dynamics in low-temperature grown gallium arsenidexe2x80x9d, Appl. Phys. Lett. 69 (17), Oct. 21, 1996, page 2566 ff.). The non-saturable absorption losses are attributed to a transition between neutral antisites (for example, ASGa0) in the semiconductor material (for example, GaAs) and the band conditions, which are 0.7 eV above the lower end of the conductive band. Because of the high neutral antisite concentration and the high density of the final conditions, this transition can only be saturated at very high pulse fluences. The Be doping in accordance with the invention, resp., the thermal annealing in accordance with the invention now considerably reduce the concentration of the neutral antisites by changing the charge condition of the defects, resp., by precipitation. As a result of this, the transition between the neutral antisites and the conduction band can be at least partially saturated, which leads to the reduction of the non-saturable absorption losses and to an increase of the absorption modulation.